1. Field of the Invention
The present invention relates to electric drives and is directed particularly to start, braking and speed regulation of induction motors, as well as to synchronous motors start, which are started by the method of the asynchronous start.
2. State of the Art
The main requirements of the methods and devices for the induction motor start, and also asynchronous start of the synchronous motors, are:                relatively low start current.        
It is known that the high start current of the motor requires excessive energy supply and, respectively, more powerful feeding equipment used only at the moment of the motor's start. Besides, the high start current shortens lifetime of the motor because of electrical and mechanical impacts at the moment of the start, and also leads to the essential overheat of the motor and to the extra consumption of the energy under the high repetitions of the starts.                the start torque, high enough to ensure the possibility of the start of the loaded motor.        the smoothness of the start process without fluctuations and spikes of currents, voltages and torques.        the absence of the device influence on the motor operation and feeding network after the motor's start process is finished.        the simplicity, reliability and economic efficiency of the device.        
The “Soft Start” of the induction motor is a well known and widely used method in practice. Hereby, the start process of the motor, the voltage value feeding the motor is gradually increased up to its rated value.
The main disadvantage of this method, under the condition of the permissible start current supply, is a low start torque that does not allow using it under relatively high load of the motor. This method is used, as a rule, for the start of an unloaded motor and requires additional equipment for the subsequent application of the load.
Another well known method of the induction motor start is a method in which the frequency and the value of the feeding voltage in the process of the start are increased up to their rated values (U.S. Pat. No. 4,357,655). In order to implement this method the device includes the inverter with the direct current intermediate link.
The disadvantage of this device is a relative complexity of the inverter design, and the device reliability is not high enough, caused by the necessity of the forced turning off of its power elements.
In addition, after completion of the start process, the inverter negatively influences the power characteristics of the motor in the operating regime, namely, it leads to the significant losses of the energy in the motor and in the inverter, to reduction of the electromagnetic torque of the motor and to the impossibility of the energy recuperation of the motor, that in turn reduces the economic efficiency of this method and device.
The expensive and large electromechanical contactors are required after the completion of the start process in order to turn off the inverter, that essentially complicates the device, enlarges its size and decreases the economic efficiency.
There is also known method of the induction motor start (R. BRZESINSKY, V. I. CHRISANOV. “Reliable and Intelligent Technology of Soft-Starting Electrical Machines”. Proceeding of the 5th UEEES'01, 2001, Szchecin, Poland, supplement vol., pp 179-182.) in which the motor feeding voltage frequency increases up to the supplied voltage frequency. The low frequency voltage is formed by means of the sequence of half waves of the supplied voltage. The amount of half waves for each frequency is changed discretely by the preset ratio.
This method avoids the disadvantages mentioned in two previous methods, however it does not provide the smoothness of the start process, i.e. the spikes and fluctuations of currents, voltages and torques because of discrete frequency change, are present.